The present invention relates generally to systems which calculate the effects of the atmosphere on the transmission of radio and optical beams, and more specifically to a moderate resolution propagation model of the earth's atmosphere. This model can be used for predicting atmospheric transmittance and background radiance from 0 to 50,000 cm.sup.-1 at a resolution of 2 cm.sup.-1 .
The interest in atmospheric transmittance and background radiance along with the associated subject of astronomical refraction goes back to Laplace. With the advent of large telescopes and phase array radar systems, this interest has become ubiquitous, since the output signals of these systems experience attenuation due to atmospheric particles, water vapor and other gases along the viewing path.
The transmittance and radiance along a path through the atmosphere depend upon the total amount and the distribution of the absorbing or scattering species as well as the variation of pressure and temperature along the path. The integrated amount of absorber or scatterer along a path is known by various names, including "column density," "equivalent absorber amount," and "air mass." While the term "air mass" applies specifically to the total amount of gas along the path, it will be used here to refer loosely to the integrated amounts for all the different species relative to the amount for a vertical path.
The task of ascertaining atmospheric transmittance and atmospheric background radiance is alleviated, to some extent by the systems disclosed in the following U.S. Patents, the disclosures of which are incorporated herein by reference:
U.S. Pat. No. 4,010,357 issued to Joseph L. Horner;
U.S. Pat. No. 4,521,861 issued to Russell H. Logan et al;
U.S. Pat. No. 5,075,856 issued to Francis Kneizys et al; and
U.S. Pat. No. 3,986,002 issued to Dorian A. DeMaio.
Perhaps the most significant of the above-cited references is the Kneizys patent; which was filed on Jul. 17, 1989 as U.S. patent application Ser. No. 07/383,372 for the LOWTRAN 7 model of the atmosphere, which is discussed below. The Holyer reference discloses a satellite method for measuring sea surface temperature which utilizes LOWTRAN 5, which is a predecessor of the present invention.
In February of 1980, the Air Force Geophysics Laboratory of Hanscom Air Force Base, Massachusetts developed LOWTRAN 5, a Fortran computer code designed to calculate atmospheric transmittance and radiance for a given atmospheric path at low spectral resolution. The details of LOWTRAN 5 are described in a technical report by F. Kneizys et al entitled "Atmospheric Transmittance/Radiance; Computer Code LOWTRAN 5, AFGL-TR-80-0067," the disclosure of which is incorporated herein by reference. This report is available from the National Technical Information Service where it is identified as document number ADA088215.
In LOWTRAN 5, 6 and 7, the atmosphere is modeled as a set of spherically symmetric shells with discrete boundaries. The temperature, pressure, and absorber (gas and aerosol) densities are specified at the layer boundaries. Between boundaries, the temperature profile is assumed linear while the pressure and densities are assumed to follow exponential profiles.
LOWTRAN 6 was developed and described in August 1983 in a technical report entitled "Atmospheric Transmittance/Radiance; Computer Code LOWTRAN 6, AFGL-TR-83-0187," the disclosure of which is incorporated herein by reference. This report is available from the National Technical Information Service, where it is identified as document number ADA137796.
LOWTRAN 6 was an improvement over the previous model LOWTRAN 5, which assumed that the index of refraction was constant between layer boundaries. LOWTRAN 6 assumes a continuous profile for the refractive index, with an exponential profile between layer boundaries. It is more accurate than the previous models and works for all paths.
The LOWTRAN 7 model and computer code calculates atmospheric transmittance and background radiance for a given atmospheric path at low spectral resolution. This version is an extension and update of the current code, LOWTRAN 6 (and its predecessors LOWTRAN 5, LOWTRAN 4, LOWTRAN 3 and LOWTRAN 2). All the options and capabilities of the LOWTRAN 6 code have been retained, but additional refinements have been added, as described below.
The LOWTRAN 7 code calculates atmospheric transmittance, atmospheric background radiance, single scattered solar and lunar radiance, direct solar irradiance, and multiple scattered solar and thermal radiance. The spectral resolution of the model is 20 cm.sup.-1 (full width at half-maximum) in steps of 5 cm.sup.-1 from 0 to 50,000 cm.sup.-1 (0.2 um to infinity). A single parameter band model is used for molecular line absorption and the effects of molecular continuum-type absorption; molecular scattering, aerosol and hydrometeor absorption and scattering are included. Refraction and earth curvature are considered in the calculation of the atmospheric slant pat and attenuation amounts along the path. Representative atmospheric, aerosol, cloud, and rain models are provided in the code with options to replace them with user-provided theoretical or measured values.
In view of the foregoing discussion, it is apparent that there remains an ongoing need to obtain refined estimates of atmospheric transmittance and background radiance, and that state-of-the-art methods are literally adapted for use almost as fast as they are developed by users that include the United States Air Force and other DOD agencies. The present invention is intended to satisfy that need.